When semiconductor devices are manufactured, a silicon oxynitride film may be formed on a semiconductor substrate to use it as a reflection preventing film. The silicon oxynitride film may he formed by plasma chemical vapor deposition (CVD) or low pressure chemical vapor deposition (LP-CVD). In addition, a hydrofluoric acid treatment is used to remove etching residues or to remove a natural oxide film on a wafer by immersing the wafer in a hydrofluoric acid solution made of hydrofluoric acid (HF) and water (H2O).
Japanese Patent Application Publication No. 59-90942 discloses a technique of leaving a protection oxide film without etching until LOCOS oxide film is grown on a silicon substrate, and washing the surface of the silicon substrate that is covered with a hydrophilic silicon nitride film and the protection oxide film. The technique restrains the generation of washing residues.
However, in a case where the surface of the silicon oxynitride film is subjected to the hydrofluoric acid treatment after the formation of the silicon oxynitride film, defects may be formed on the surface of the silicon oxynitride film.